1. Field of Invention
The present invention relates to silver halide emulsions and methods for producing same and, particularly, pertains to a silver halide emulsion, which has excellent reciprocity law characteristics, high sensitivity and high contrast, and a method for producing same.
2. Description of the Related Art
As for silver halide emulsions for use in a color photographic paper and the like, a silver halide emulsion which has a high silver chloride content (hereinafter referred to also as “high silver chloride emulsion”) has ordinarily been used to satisfy a request for performing rapid processing to enhance productivity. Low sensitivity softening is generally easily caused in these halogen emulsions, which have a high silver chloride content, by high illuminance exposure, such as a laser exposure. Moreover, a fogging density in these halogen emulsions is high. Various techniques have been disclosed to improve these points.
On the other hand, for improving high intensity reciprocity failure of a silver chloride emulsion, a technique for doping iridium is known. However, it is known that the silver chloride emulsion doped with iridium causes latent image sensitization for a short period of time after exposure. For example, JP-B No. 7-34103 discloses providing a local phase which is high in silver bromide and, then doping the thus-provided local phase with iridium to solve the problem of latent image sensitization. The silver halide emulsion prepared in this manner has high sensitivity and high contrast in a relatively high intensity exposure of about {fraction (1/100)} second and does not cause a problem of latent image sensitization; however, it has come to be apparent that, when it is intended to maintain high sensitivity even for a super high intensity exposure of 1 microsecond which is required by a digital exposure performed by a laser-scanning exposure, a problem occurs, such that it is unlikely to cause a high contrast gradation. U.S. Pat. No. 5,691,119, discloses a method of making a high intensity gradation to have high contrast using a method of preparing an emulsion having a localized phase which is high in a silver bromide content; however, there is a defect that an effect thereof is not sufficient and also performance is not stable in repeated preparation of the emulsion.
U.S. Pat. Nos. 5,726,005 and 5,736,310 discloses that an emulsion which is high in sensitivity and low in high intensity reciprocity failure can be obtained by preparing a silver chloride-rich emulsion which contains a maximum concentration of iodine on a sub-surface thereof. Although such emulsion can certainly can obtain a sensitivity as high as that of the high intensity exposure, it was found that the emulsion provides gradation which is consistently low in contrast, is not appropriate for a digital exposure in which a dynamic range of light amount is limited and, further, provides a high fogging density whereby the emulsion is not appropriate as a material to be used, without further modification, for prints. U.S. Pat. Nos. 5,728,516, 5,547,827 and 5,605,789; and JP-A No. 8-234354 disclose methods for reducing the fogging density of an emulsion which contains a maximum concentration of iodine on a sub-surface thereof; however, none of these methods exhibites an effect sufficient to be used as a material for prints.
JP-A Nos. 58-95736, 58-108533, 60-222844, 60-222845, 62-253143, 62-253144, 62-253166, 62-254139, 63-46440, 63-46441 and 63-89840; and U.S. Pat. Nos. 4,820,624, 4,865,962, 5,399,475 and 5,284,743 disclose that high sensitivity can be obtained by localize-adding phases of varied states, each of which has a high silver bromide content, into high silver chloride emulsions, in various forms.
As described above, providing a high bromide-containing phase to grains has been disclosed for the purpose of improving reciprocity law characteristics, and obtaining high sensitivity and high contrast of the high silver chloride emulsion. However, in methods so far disclosed, a distribution among grains of a bromide content and an uneven distribution of a high bromide-containing phase occur along with recrystallization and, as a result, it was difficult to obtain photographic characteristics with an emulsion having high sensitivity and high contrast or to prepare an emulsion which has excellent reciprocity law characteristics and does not cause the latent image sensitization.